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1. Co-intercalation-free ether electrolytes for graphitic anodes in lithium-ion batteries

   https://doi.org/10.1039/D2EE01489K  

   Ma, Peiyuan; Mirmira, Priyadarshini; Eng, Peter J.; Son, Seoung-Bum; Bloom, Ira D.; Filatov, Alexander S.; Amanchukwu, Chibueze V. (November 2022, Energy & Environmental Science)

   Carbonate-based electrolytes are widely used in Li-ion batteries but are limited by a small operating temperature window and poor cycling with silicon-containing graphitic anodes. The lack of non-carbonate electrolyte alternatives such as ether-based electrolytes is due to undesired solvent co-intercalation that occurs with graphitic anodes. Here, we show that fluoroethers are the first class of ether solvents to intrinsically support reversible lithium-ion intercalation into graphite without solvent co-intercalation at conventional salt concentrations. In full cells using a graphite anode, they enable 10-fold higher energy densities compared to conventional ethers, and better thermal stability over carbonate electrolytes (operation up to 60 °C) by producing a robust solvent-derived solid electrolyte interphase (SEI). As single-solvent–single-salt electrolytes, they remarkably outperform carbonate electrolytes with fluoroethylene carbonate (FEC) and vinylene carbonate (VC) additives when cycled with graphite–silicon composite anodes. Our molecular design strategy opens a new class of electrolytes that can enable next generation Li-ion batteries with higher energy density and a wider working temperature window. 

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2. In Situ Engineering of Inorganic‐Rich Solid Electrolyte Interphases via Anion Choice Enables Stable, Lithium Anodes

   https://doi.org/10.1002/adma.202305645  

   Weeks, Jason_A; Burrow, James_N; Diao, Jiefeng; Paul‐Orecchio, Austin_G; Srinivasan, Hrishikesh_S; Vaidyula, Rinish_Reddy; Dolocan, Andrei; Henkelman, Graeme; Mullins, C_Buddie (December 2023, Advanced Materials)

   Abstract The discovery of liquid battery electrolytes that facilitate the formation of stable solid electrolyte interphases (SEIs) to mitigate dendrite formation is imperative to enable lithium anodes in next‐generation energy‐dense batteries. Compared to traditional electrolyte solvents, tetrahydrofuran (THF)‐based electrolyte systems have demonstrated great success in enabling high‐stability lithium anodes by encouraging the decomposition of anions (instead of organic solvent) and thus generating inorganic‐rich SEIs. Herein, by employing a variety of different lithium salts (i.e., LiPF_6,LiTFSI, LiFSI, and LiDFOB), it is demonstrated that electrolyte anions modulate the inorganic composition and resulting properties of the SEI. Through novel analytical time‐of‐flight secondary‐ion mass spectrometry methods, such as hierarchical clustering of depth profiles and compositional analysis using integrated yields, the chemical composition and morphology of the SEIs generated from each electrolyte system are examined. Notably, the LiDFOB electrolyte provides an exceptionally stable system to enable lithium anodes, delivering >1500 cycles at a current density of 0.5 mAh g⁻¹and a capacity of 0.5 mAh g⁻¹in symmetrical cells. Furthermore, Li//LFP cells using this electrolyte demonstrate high‐rate, reversible lithium storage, supplying 139 mAh g_(LFP)⁻¹at C/2 (≈0.991 mAh cm⁻², @ 0.61 mA cm⁻²) with 87.5% capacity retention over 300 cycles (average Coulombic efficiency >99.86%). 

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3. Use of Ethylene Carbonate Free Ester Solvent Systems with Alternative Lithium Salts for Improved Low-Temperature Performance in NCM622∣∣ Graphite Li-ion Batteries

   https://doi.org/10.1149/1945-7111/ac9d0a  

   Rodrigo, Nuwanthi D.; Jayawardana, Chamithri; Rynearson, Leah; Hu, Enyuan; Yang, Xiao-Qing; Lucht, Brett L. (November 2022, Journal of The Electrochemical Society)

   An investigation of alternative lithium salts, lithium tetrafluoroborate (LiBF 4 ), lithium difluoro(oxalato)borate (LiDFOB) and lithium hexafluorophosphate (LiPF 6 ), in novel ester-based (methyl acetate/fluoroethylene carbonate- MA/FEC or methyl propionate/fluoroethylene carbonate- MP/FEC) electrolyte formulations has been conducted in LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622)/graphite cells to improve low temperature cycling performance of lithium ion batteries at −20 °C. Improved low temperature performance was observed with all the lithium salts in MA/FEC electrolyte while comparable room temperature (25 °C) capacities were observed with LiPF 6 salt only. Detailed ex-situ analysis of surface films generated with LiBF 4 , LiDFOB and LiPF 6 in ester-based electrolytes reveals that the solid electrolyte interphase (SEI) is predominately composed of lithium salt decompaction products and addition of 10% FEC (by volume%) may not be sufficient at forming a protective SEI. 

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4. In situ formation of a multicomponent inorganic-rich SEI layer provides a fast charging and high specific energy Li-metal battery

   https://doi.org/10.1039/C9TA05063A  

   Sun, Ho-Hyun; Dolocan, Andrei; Weeks, Jason A.; Rodriguez, Rodrigo; Heller, Adam; Mullins, C. Buddie (July 2019, Journal of Materials Chemistry A)

   The performance of the rechargeable Li metal battery anode is limited by the poor ionic conductivity and poor mechanical properties of its solid-electrolyte interphase (SEI) layer. To overcome this, a 3 : 1 v/v ethyl methyl carbonate (EMC) : fluoroethylene carbonate (FEC) containing 0.8 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 0.2 M lithium difluoro(oxalate)borate (LiDFOB) dual-salts with 0.05 M lithium hexafluorophosphate (LiPF 6 ) was tested to promote the formation of a multitude of SEI-beneficial species. The resulting SEI layer was rich in LiF, Li 2 CO 3 , oligomeric and glass borates, Li 3 N, and Li 2 S, which enhanced its role as a protective yet Li + conductive film, stabilizing the lithium metal anode and minimizing dead lithium build-up. With a stable SEI, a Li/Li[Ni 0.59 Co 0.2 Mn 0.2 Al 0.01 ]O 2 Li-metal battery (LMB) retains 75% of its 177 mA h g −1 specific discharge capacity for 500 hours at a coulombic efficiency of greater than 99.3% at the fast charge–discharge rate of 1.8 mA cm −2 . 

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5. Fluorinated ethylene carbonate as additive to glyme electrolytes for robust sodium solid electrolyte interface

   https://doi.org/10.1016/j.xcrp.2023.101356  

   Sarkar, Susmita; Lefler, Matthew J.; Vishnugopi, Bairav S.; Nuwayhid, R. Blake; Love, Corey T.; Carter, Rachel; Mukherjee, Partha P. (April 2023, Cell Reports Physical Science)

   The utilization of alkali metal anodes is hindered by an inherent instability in organic electrolytes. Sodium is of growing interest due to its high natural abundance, but the carbonate electrolytes popular in lithium systems cannot form a stable solid electrolyte interphase (SEI) with a sodium electrode. Using half-cell and symmetric-cell analysis, we identify specific glyme (chain ether) electrolytes that produce thin, predominantly inorganic SEI at the sodium metal interface, and we study the effect of ethylene carbonate and fluoroethylene carbonate (FEC) additives on the SEI formed in these systems via X-ray photoelectron spectroscopy. Through in situ optical microscopy, we observe the onset and growth of sodium dendrites in these electrolytes. We determine that the SEI formed by glyme alone may not support extensive or extreme cycling conditions, but the addition of FEC provides a more robust SEI to facilitate numerous consistent sodium plating and stripping cycles. 

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